That's right. You're suggesting instance chains [1][2]. [1]: https://ghc.haskell.org/trac/ghc/ticket/9334 [2]: http://web.cecs.pdx.edu/~mpj/pubs/instancechains.pdf
Richard On Jun 7, 2015, at 11:12 AM, Dan Doel <dan.d...@gmail.com> wrote: > It seems to me the problem is that there's no way to define classes by > consecutive cases to match the family definitions. I don't know what a good > syntax for that would be, since 'where' syntax is taken for those. But it > seems like it would correspond fill the hole here. > > On Sun, Jun 7, 2015 at 7:27 AM, Richard Eisenberg <e...@cis.upenn.edu> wrote: > This is all expected behavior. GHC's lazy overlap checking for class > instances simply cannot apply to type families -- it would be unsound. I'm > afraid I don't see what can be improved here. > > Richard > > On Jun 6, 2015, at 2:04 AM, AntC <anthony_clay...@clear.net.nz> wrote: > > >> From: AntC > >> Date: 2015-06-04 22:39:25 GMT > >> > >> Take the standard example for partial overlaps. > >> Suppose I have a class: ... > > > >> I'm also getting (in more complex examples) > >> GHC complaining it can't infer the types > >> for the result of f. > >> So now I'm having to put type equality > >> constraints on the class instances, > >> to assure it that F comes up with > >> the right type. > > > > In a reduced example, I'm still getting > > poor type checking. This is GHC 7.8.3. > > This seems so dumb, I'm suspecting a defect, > > It's similar to > > but much more glaring than: > > https://ghc.haskell.org/trac/ghc/ticket/10227 > > https://ghc.haskell.org/trac/ghc/ticket/9918 > > > > {-# LANGUAGE TypeFamilies, > > FlexibleInstances > > #-} > > module ClosedTypeFamily where > > > > data Foo b c = Foo b c deriving (Eq, Read, Show) > > > > type family F a where > > F (Foo Int c) = Int -- Foo Int is first instance > > F (Foo b Char) = Char > > > > class C a where f :: a -> F a > > > > instance C (Foo Int c) where -- compiles OK > > f (Foo x _) = x > > > > instance (F (Foo b Char) ~ Char) => C (Foo b Char) where > > f (Foo _ y) = y > > > > needs the eq constraint. Without it, GHC complains: > > Couldn't match expected type ‘F (Foo b Char)’ > > with actual type ‘Char’ > > Relevant bindings include > > f :: Foo b Char -> F (Foo b Char) > > In the expression: y > > In an equation for ‘f’: f (Foo _ y) = y > > > > Note that if I change the sequence > > of the family instances for F, > > then GHC instead complains > > about the class instance for (Foo Int c). > > > > OK these are overlapping class instances. > > But GHC's usual behaviour > > (without closed type families) > > is to postpone complaining > > until and unless a usage > > (Foo Int Char) actually turns up. > > > > BTW if I put a first family instance > > F (Foo Int Char) = Int > > to explicitly catch the overlap, > > then GHC complains about **both** class instances. > > > > Reminder [to Richard] > > I need not only types but also terms. > > > > AntC > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgow-haskell-users@haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > > > > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >
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